Air abrasion tools are becoming increasingly more popular for use on, for example, dental patients where an abrasive-laden fluid, such as air containing microscopic non-toxic abrasive powder, is directed onto one or more of the patient's teeth for quickly removing decay, preparing the teeth to receive fillings and/or for cleaning the teeth. Such abrasion devices provide advantages over conventional dental drills. For example, material is removed by pressurized abrasive air eliminating the heat, noise, and vibration produced by high speed drills. Also, the need for anesthesia is reduced because fluid used to cool the drill is eliminated. Furthermore, the risk of saliva contamination is reduced by maintaining a dry field. Air abrasion devices use a narrowly focused stream of particle-laden air that removes material from the tooth in proportion to various factors, such as, for example, the size and nature of the particle, the velocity of the particle on impact, and incident angle of impact of the particle. Because the cutting ability of the abrasive particle within the air stream is a function of the velocity of the particle, which in turn is a function of the air stream velocity, it is desirable to produce an air stream with very high velocity. Thus, the most effective method of increasing the material removal efficiency of the abrasive particles is to increase the air stream velocity.
Currently, the manufacturers of devices that accelerate particles to high speeds for abrasion and/or cutting on, for example, the tooth of a dental patient, use converging nozzles or constant area nozzles. Converging area or constant area nozzles can at best produce sonic flow velocities inside the nozzle and only slightly supersonic velocities just past the exit plane of the nozzle. For example, converging or constant area nozzles may produce a relatively low supersonic flow velocity of about Mach 1.2 for an extremely short distance past the exit plane of the nozzle. The velocity of the abrasive particles is controlled by the pressure difference across the nozzle. In order to achieve such a velocity of about Mach 1.2, a pressure of about 160 psig must be used. Most common dental office or household equipment can provide a reservoir gage pressure of up to about 80 psig. In order to achieve 160 psig, it is necessary to have an additional heavy duty compressor. This increases costs and takes up a considerable amount of space.
Another disadvantage of using such high pressure is that as the abrasive air fluid exhausts from the nozzle, the immediate drop in pressure causes the fluid to decrease in temperature. The static temperature of the fluid can decrease to, for example, about 20.degree. Fahrenheit. Air flow of this temperature against a patent's tooth can cause extreme discomfort. In order to compensate for the coldness of the airstream and to increase patient comfort, an additional heater may be needed in order to heat the air. Another alternative to compensate for the coldness of the air stream would be to use an anesthetic which usually must be injected with a hypodermic needle.
These disadvantages can be overcome by the use of a converging-diverging (CD) nozzle. A CD nozzle consistently produces supersonic fluid velocities substantially above Mach 1 with a typical in-house source of pressurized air and at a temperature comfortable to the patient.
Converging-diverging nozzles have been known in heavy industry applications. For example, U.S. Pat. No. 5,390,450 discloses a supersonic converging-diverging exhaust nozzle to expel liquid CO.sub.2 for cleaning a printed circuit board. This device coagulates the CO.sub.2 snow into larger CO.sub.2 snow particles and uses a supersonic nozzle operated in the overexpanded mode to focus the CO.sub.2 snow onto the workpiece while reducing the noise produced by the pressurized exhaust.
U.S. Pat. No. 5,283,985 discloses a method of impacting abrasive particles against a surface to be treated using an internal burner by introducing the abrasive particles into the supersonic jet stream after expansion of combustion gases from the internal burner to nearly atmospheric pressure from very high pressures, and by causing the abrasive particles to accelerate through a nozzle having a length long enough to accelerate the particles to a much greater impact velocity.
U.S. Pat. No. 4,633,623 discloses a sandblasting nozzle to decontaminate radioactive members by means of a jet formed from a mixture of water and abrasive particles.
U.S. Pat. No. 5,283,990 discloses a sandblasting device that produces less turbulence as the blast media particles are accelerated through the nozzle that maintains maximum velocity of the blast media particles and cleaning rate during operation.
U.S. Pat. No. 5,275,486 discloses a nozzle in which a two-phase mixture of two fluids is accelerated by an expanded portion of the nozzle to supersonic velocity creating a one-phase mixture.
U.S. Pat. No. 5,050,805 discloses a supersonic nozzle in which the sound emitted by the nozzle is reduced to as low a level as possible to permit safe operation.
These prior devices are used in industrial applications such as sandblasting and operate at high pressures and/or at temperatures that are not applicable for use on biological organisms such as humans, plants and animals.